A liquid crystal display device (LCD) is a display device that expresses pixels on the principle of selectively penetrating light depending on the changes in polarization effects according to the array of liquid crystal disposed between polarizing plates.
IF the display device consists of the liquid crystal and polarizing plate comprising only polarizer, brightness or contrast is seriously deteriorated, or light leakage is caused when a liquid crystal is viewed at various viewing angles except when the liquid crystal is viewed vertically from the front side of a liquid crystal screen. Therefore, the polarizing plate includes an optical compensation film, such as a retardation film or a viewing angle compensation film, in addition to the polarizer, or these films are generally adhered to a panel through a separate process.
The optical compensation film is mainly divided into two films: one stretched film prepared by stretching a polymer film to give optical anisotropy, and the other liquid crystal film prepared by coating a plastic substrate with a polymeric liquid crystal compound and curing the polymeric liquid crystal compound. In particular, the stretched film does not have anisotropy such as optically uniaxial or biaxial anisotropy, but the liquid crystal film may have various optical properties that the aligned film may not have. Liquid crystal may be mainly divided into a disc-type liquid crystal and a rod-type liquid crystal, depending on the shape of the liquid crystal molecules. Among them, the rod-type liquid crystal can be aligned in various shapes such as planar, homeotropic, tilted, splay, cholesteric shapes, and therefore their optical properties owing to the various shapes are also diverse and unique, compared to those of the aligned film. Also, the liquid crystal film may function as a protective film and an optical compensation film of the polarizer by forming the liquid crystal film on the aligned film. Accordingly, the liquid crystal film has been increasingly demanded.
Particularly in the TN-mode liquid crystal display device, the liquid crystal film may be used as the optical compensation film when the liquid crystal film is splay-aligned. That is, the splay-aligned liquid crystal film may become an optical compensation film for compensating for light leakage or phase difference (viewing angle). Here, when a TAC film used as the protective film of the polarizer is coated with the splay-aligned liquid crystal film, which is later used as the protective film of the polarizer, the protective film may be used as an integrated polarizing plate of the optical compensation film. Generally, an optical compensation film including the splay-aligned liquid crystal film is referred to as an O-plate.
In this case, the splay-aligned liquid crystal film is generally manufactured by coating a plastic substrate with a composition for an alignment film that is used to form a liquid crystal alignment film, drying and curing the composition to form an alignment film, after then rubbing the alignment film to give an alignment property, and coating the alignment film with a liquid crystal, following by repeating the drying and curing process. If the alignment film does not have excellent adhesiveness to the liquid crystal film or the substrate, the liquid crystal film is peeled off from the alignment film, or the liquid crystal film may be shrunken under hot and humid environments. Accordingly, the liquid crystal film with excellent adhesiveness is required.
Polyimide, as a material for an alignment film that is used to form the alignment film, was widely known in the art, but polyvinyl alcohol has been recently used. However, when the polyvinyl alcohol is used in the composition for an alignment film, a liquid crystal film may be peeled off from the alignment film due to the insufficient adhesive force to the liquid crystal film, or the liquid crystal film may be shrunken under hot and humid environments, as described above.
Also, Japanese Patent Laid-open Publication Nos. 2002-62426 and Hei7-179125 disclose a method using a modified polyvinyl alcohol in which polyvinyl alcohol is modified to improve an adhesive force to a liquid crystal film, but it is difficult to directly use the modified polyvinyl alcohol as a coating solution, and therefore the modified polyvinyl alcohol is subject to additional separation and purification process, and then may be used as a composition for an alignment film.
Also, the alignment film is generally formed on a substrate. Since even the composition for an alignment film prepared thus may be restrictively used according to the kind of available substrates, the alignment film may be effectively adhered to the substrate such as triacetyl cellulose (TAC), but adhesiveness to the substrate such as cycloolefin polymer (COP) is poor.
That is, a triacetyl cellulose substrate is widely used to form an alignment film, but has problems that light leakage appears in the substrate under hot and humid environments due to its high hygroscopicity, and its degree of polarization is deteriorated. Therefore, there have been proposed a variety of substrates that may be substituted for the triacetyl cellulose substrate, and a cycloolefin polymer is one representative substrate among them. Here, the cycloolefin polymer may function as at least one of an A-plate (A-plate) and a negative C-plate, as well as a protective film of a polarizer. Accordingly, the cycloolefin polymer has excellent durability under the conditions of high temperature and high humidity, compared to the polarizing plate where triacetyl cellulose is used as a protection layer.
However, the cycloolefin polymer has poor adhesiveness when the conventional composition for an alignment film is used for cycloolefin polymer as described above, and therefore an alignment film may be detached from the cycloolefin polymer.
In addition to the context as described above, the alignment film should also have an basic ability to uniformly align liquid crystal molecules in a rubbing direction when a surface of the alignment film is rubbed, and this ability of the alignment film may be represented by an optical-axis departure angle showing a degree where the liquid crystal molecules depart from the rubbing direction. However, the convention alignment film, namely an alignment film using polyvinyl alcohol, has a alignment to prevent some degree of optical-axis departure, but the degree of optical-axis departure of the alignment film is not sufficient. If a liquid crystal has a high optical-axis departure angle, a polarizing plate made of the liquid crystal has a problem that light leakage may be included and optical loss such as poor contrast ratio may be caused in the polarizing plate.
Accordingly, a retardation film in which the liquid crystal film is well adhered and the liquid crystal film adhered to the alignment film has an excellent alignment property, still remains to be developed.
Also, the polarizing plate generally includes protective films in both sides of a polarizer, and optical compensation films such as a retardation film or a viewing angle compensation film is generally included in the polarizing plate in addition to the protective films. However, layers, which are adhered to a display device, such as a polarizing plate are desirably decreased in thickness as far as possible since the manufacturing cost is higher with an increasing thickness of the polarizing plate, and the increased thickness of the polarizing plate is an obstacle to the display device.